1. Field of the Invention
The present invention relates to a drive system control method and a drive power transmission control system for a vehicle provided with a clutch device, variable in connection force, between a front wheel torque transmission axle and a rear wheel torque transmission axle.
2. Discussion of the Related Art
Heretofore, as described in Japanese unexamined, published patent application No. 7-186766 (186766/1995) for example, there have been known four-wheel drive vehicles of the construction that the connection force of a clutch device provided between a front wheel torque transmission axle and a rear wheel torque transmission axle is kept zero for a predetermined time period subsequence to the starting of the vehicle to make either of front wheels and rear wheels serve as driven wheels and is heightened to bring the vehicle into a four-wheel drive state after the expiration of the predetermined time period.
However, in the four-wheel drive vehicle, either the front wheels or the rear wheels may slip when the four-wheel drive vehicle begins to start, and the connection force of the clutch device between the front wheel torque transmission axle and the rear wheel torque transmission axle is controlled to be heightened for suppression of the slip. This may result in ending the slip to enable the vehicle to start or in causing the both of the front and rear wheels to slip further. In the latter case, some torsion is accumulated on the torque transmission axles while either the front wheels or the rear wheels are slipping but the other wheels are not slipping. When the other wheels slip with the torsion being accumulated on the torque transmission axles, the friction of those wheels with the road surface is changed from a static friction state to a dynamic friction state. This disadvantageously causes the resilient force produced by the torsion to be freed instantaneously, whereby the torque transmission axles are vibrated in the circumferential direction. As a consequence, the driver is given an unpleasant feeling or an anxious feeling.